Current limiter unit for molded case circuit breakers

ABSTRACT

A self-contained current limiter unit allows lower short-circuit-rated circuit breakers to be utilized within industrial applications. The unit is electrically-connected in series with the circuit breakers and the protected load to limit the short circuit current to a fixed predetermined value while the circuit breaker trip unit responds to interrupt the circuit current. A single unit can be arranged in electrical series connection with a plurality of lower short-circuit-rated circuit breakers to allow a substantial savings in component and equipment costs.

BACKGROUND OF THE INVENTION

Industrial-rated circuit breakers such as described within U.S. Pat. No.4,782,583 entitled "Method of Assembly of a Molded Case Circuit BreakerCrossbar" require operating components that are sized to handle thelargest available current during circuit interruption. Since a shortcircuit available current is only limited by the residual circuitimpedance, the circuit breakers within industrial environments are sizedto handle short circuit currents which are several orders of magnitudegreater than faults occurring because of routine circuit faults that aredesignated as "long time" or "short time" faults to distinguish from theshort circuit faults.

Several attempts have been made to limit the size of the circuit breakercurrent-carrying components by limiting the short circuit current to amanageable level. One such attempt is in the "arcless" circuitinterrupter described within U.S. Pat. No. 4,583,146 entitled "FaultCurrent Arcless Circuit Interruption" wherein the current is transferredto a voltage variable resistor before contact separation to eliminatethe arc that occurs upon contact separation and to rapidly separate thecontacts during the early stages of the current waveform.

U.S. Pat. No. 4,375,021 entitled "Rapid Electric Arc Extinguishing"describes the use of so-called "blow open" contact arms to limit thecircuit overload current to a reasonable value prior to the articulationof the circuit breaker operating mechanism to interrupt the circuitcurrent.

U.S. Pat. No. 3,488,761 entitled "Current Limiter Device" describes thecombination of a positive temperature coefficient material (PTC) inparallel with a metal resistor and in series with a pair of separablecontacts to limit the current that is interrupted by the contacts.

U.S. Pat. No. 4,413,301 entitled "Circuit Protection Devices" describesthe use of a PTC device in series with a metallic resistor and theseparable contacts.

One object of the present invention is to describe a current limiterunit employing a PTC device to assist in limiting short-circuit currentto a predetermined value to allow circuit breakers of low short-circuitratings to be employed within higher rated electrical distributioncircuits. Another object of the present invention is to describe acurrent limiter unit that avoids expenditure of electric power duringquiescent current conditions.

SUMMARY OF THE INVENTION

The invention comprises a current limiter unit in the form of a pair ofseparable contacts connected in parallel with a metallic resistor and aPTC resistor. The current limiter holds the current to a reasonablelevel upon short circuit to then allow a low short-circuit rated circuitbreaker to interrupt the circuit current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front plan view of a service entrance enclosure containingthe current limiting unit in accordance with the invention;

FIG. 2 is an enlarged top perspective view of the current limiter unitof FIG. 1 with the cover removed to detail the operating components;

FIG. 3 is diagrammatic representation of the circuit components withinthe transfer circuit of FIG. 2; and

FIG. 4 is a graphic representation of the current transport within thecurrent limiter of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A residential circuit breaker load center 10 is depicted in FIG. 1 andincludes an enclosure 11 containing a front access door 12. A series oflow ampere rated residential circuit breakers 13 are arranged within theenclosure and are turned ON and OFF by means of operating handles 14accessible through slots 15 formed within the door 12. One example of alow short-circuit rated residential circuit breaker is found within U.S.Pat. No. 4,513,268 entitled "Automated Q-Line Circuit Breaker". Thecircuit breakers individually connect between the service entrance lineconductors and the associated loads (not shown). The current limiterunit 16 is electrically connected in series with the circuit breakerssolely to limit the circuit current upon short circuit overcurrentconditions and to allow the circuit breakers to interrupt the circuit.

The current limiter 16 is depicted in FIG. 2 to detail the separablecontact arrangement 35 consisting of the movable contact arms 23,24which are pivotally-attached at one end to the enclosure 17 as depictedat 27,28 and carry the movable contacts 25,26 at an opposite endthereof. The movable contact arms connect with the external circuit bymeans of the conductors 20, 21 when the current limiter unit iselectrically-connected in series with the circuit breakers 13 of FIG. 1by means of the exterior terminals 18, 19 arranged at opposite ends ofthe enclosure 17. The movable contact arm arrangement is similar to thatdescribed within U.S. Pat. No. 4,485,283 entitled "Current Limiter Unit"wherein the movable contact arms 23,24 are arranged within a slot motor36 consisting of a pair of transformer core laminations 37, 38 joined bya bight 39. When the current through the contact arms exceeds apredetermined value, the opposing magnetic forces generated within thecontact arms blow the contact arms apart and cause the movable contacts25,26 to become separated. Unlike the current limiter described withinthe aforementioned U.S. Pat. No. 4,485,283 a transfer circuit 22 iselectrically-connected in parallel with the movable contact arms 23,24by means of the braid conductors 29,30, and terminals 31,32 minimize theoccurrence of an arc when the movable contacts become separated.

The resistance of the movable contacts 25,26 is in the order 20 to 80 ofmicro-ohms which is relatively low compared to the higher resistancethrough the parallel transfer circuit 22. Upon contact separation, thecurrent transfers through the transfer circuit and becomes limited inthe manner to be described below in some detail.

The components within the transfer circuit 22 are shown in FIG. 3 toconsist of a PTC unit 40 electrically connected in parallel to ametallic resistor 41 by means of rails 33, 34 which terminate atterminals 31,32. The separable contact arrangement 35 is depicted inphantom to show the parallel connection with PTC and metallic resistor.The circuit current that transports between terminals 31,32 is limitedsolely by the low contact resistance described earlier. When the currentincreases by short circuit fault, the contacts separate and the currenttransfers through the parallel combination of the PTC and the metallicresistor. Since the temperature of the PTC is at a low initial value,which is lower than the so-called "cold" resistance of the metallicresistor, the current transfers mainly through the PTC which becomesheated in the process. One example of a PTC material is the conductivepolymer described within the aforementioned U.S. Pat. No. 4,413,301entitled "Circuit Protection Devices Comprising PTC Element". Once thePTC is at an elevated temperature caused by resistance heating, thecurrent then transfers to the metallic resistor which is now at a lowerresistance than the heated PTC. The metallic resistor consisting oftantalum or tungsten alloys is able to sustain the higher current andassociated temperature and forces the current to a predetermined valuedepending on the selected operating temperature of the metallicresistor. The current is then limited by the resistance value of theresistor and is prevented from further increase. The enclosure can beheat-sinked to a large metallic substrate of aluminum, copper or othermaterial of high specific heat and high thermal conductivity to insurethe predetermined operating characteristics of the current limiter unitif so desired. The automatic transfer of the current from the contactsto the PTC and metallic resistor is best seen by referring now to thecurrent profile 42 depicted in FIG. 4.

The circuit current proceeds along the profile 42 from the quiescentcurrent I₀ within the steady state design of the circuit breakers shownearlier in FIG. 1. Upon occurrence of a short circuit at t₁, the movablecontact arms 23,24 of FIG. 2 are blown open by the rising current I₁ andthe associated movable contacts 25, 26 become rapidly separated at t₂.The current then transfers to the PTC and increases along the currentramp depicted at I₂ until the current mainly transfers over to themetallic conductor at t₃ and follows the current wave form depicted atIn. The current value In represents the current value as limitedprimarly by the resistance of the PTC and continues to be limited untilthe associated circuit breaker operates to interrupt the current att_(n).

A current limiter device has been described wherein a PTC device iselectrically-connected in parallel with a pair of separable contacts anda metallic resistor. The current limiter allows low rated circuitbreakers to be employed within higher rated electric circuits bylimiting the short circuit current to a predetermined value.

I claim:
 1. A current limiter comprising:an enclosure; a pair of contactarms within said enclosure arranged within a magnetic slot motor forautomatic separation upon occurrence of overcurrent conditions throughsaid contact arms; a pair of contacts, one of said contacts beingarranged on one end of one of said contact arms for transport of circuitcurrent between said contacts upon quiescent current conditions, and fortransfer of circuit current when said contact arms become separated; aPTC device within said enclosure electrically-connected with saidcontact arms; and an ohmic resistor within said enclosureelectrically-connected with said PTC device andsaid contact arms wherebycircuit current transfers from said contacts to said PTC device and saidohmic resistor when said contacts become separated.
 2. The currentlimiter of claim 1 wherein said PTC device comprises a conductivepolymer.
 3. The current limiter of claim 1 wherein said ohmic resistorcomprises tungsten or tantalum.
 4. The current limiter of claim 1further including a circuit breaker electrically connected in serieswith said contacts, said PTC and said ohmic resistor.
 5. An economiccircuit breaker load center comprising:an enclosure; a pair of movablecontact arms within said enclosure arranged within a magnetic slot motorfor automatic separation by means magnetic repulsion upon occurrence ofovercurrent conditions through said movable contact arms; a pair ofmovable contacts, one of said movable contacts being arranged on one endof one of said movable contact arms for transport of circuit currentbetween said contacts upon quiescent current conditions, and forinterruption of circuit current when said movable contact arms becomeseparated; a PTC device within said enclosure electrically-connectedwith said moveable contact arms; an ohmic resistor within said enclosureelectrically-connected with said PTC device and said moveable contactarms whereby circuit current transfers from said contacts to said PTCdevice and said ohmic resistor when said contacts become separated and;a plurality of circuit breakers within said enclosure, each of saidcircuit breakers being arranged for connection with an associatedelectric circuit, each of said circuit breakers being electricallyconnected with said contacts, said ohmic resistor and said PTC device.